A habit I started in my Win98 days.. I carry a spare HDD (currently a 40G) .. and A norton Ghost Boot disk.. Before any major stuffing around I image the drive.. stuff around with the image.. and when the machine is working like a well oiled mechenical device.. I image back to the customers drive.. If I totaly stuff up.. I create a new image from the customers drive..
This idea maeans I can disable completly the system restore..

My advice is very much a rewording of waht has been given..

1/ back up the drive
2/ Scan disk & defrag the drive..(btw how much free space have you got if less than 10% that is your speed problem)
3/ run the usual HDD clean stuf.. empty tem internet/cookies/windows temp/recycle bin/remove un wanted progs/remove super sized files no longer needed..if you didnt do step one leave system restore where it is..for now)
4/ run adware/spyware tools.. adaware/spybot s&d/the Cleaner/check the show with HJT
5/ defrag again..is the machine running smooth? yes.. jump ahead
6/ update the Anti virus
7/ restart in safemode.. and run all the tools mentioned, ie steps 3 & 4 and then run the anti virus (I know that AVG and I have had NAV scan in safe mode)and do a HJT check..clean any crud found in any part of this step..
8/ restart in normal mode.. Defrag.. Now is it running ok..
9/ disable System restore.. restart machine
10/ renable system restore and set the space used to less than 10% of the drive.. my prefered is 7% (just because it is less than 10 but more than 5..ok)
11/ Defrag once more.. and if it has never been done a scan disk


Scan disk (I use HDD regenerator) is used on the customers HDD.. and is normaly done just before restoring the image to their original drive..

Normaly disabling the system restore deletes all the files in the restore folder.. if not clean the folder while System restore is disabled


Cheers